1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, to an improved tire exhibiting excellent performance in driving on ice and snow.
2. Description of the Related Art
The use of studded tires has been prohibited and various improvements with treads have been made to produce more improved excellent studless tires.
Such studless tires have a common characteristic in that the treads have many sipes extending straightly in the direction of the depth.
A surface of the tread is divided into many blocks by the sipes. The shape of the sipe in the transverse direction has been studied to increase the friction coefficient (xcexc) on ice by using corner portions of the blocks (the edge effect).
As conventional methods for increasing the friction coefficient on ice, there are a method in which the number of the sipe is increased and a method in which the draining performance is improved.
When the number of the sipes is increased in accordance with a conventional method, the road holding property deteriorates due to a decrease in rigidity of the block and a desired effect cannot be obtained. Moreover, the deterioration in the road holding property causes irregular wear (heel-and-toe wear).
The present invention has been made to overcome the above problem. An object of the present invention is to provide a pneumatic tire which can suppress deterioration in the road holding property caused by an increase in the number of the sipe and exhibits improvements in all of the grip performance of the tire on wet surfaces, the acceleration and braking performance on ice and resistance to irregular wear(abrasion).
In the first aspect of the present invention, the pneumatic tire comprising a tread which comprises: block-shaped land portions divided by a plurality of main grooves extending in a circumferential direction of the tire and by many lug grooves intersecting the main grooves, and a plurality of sipes formed on the block-shaped land portions; wherein at least one portion of the sipe in the direction of the depth of the sipe has an amplitude in the direction of the width of the sipe and extends in the transverse direction of the tire, and at least one portion of each sipe in the direction of the depth of the sipe is disposed at an offset position in the circumferential direction of the tire relative to an other portion of the sipe.
The portion disposed in an offset position in the circumferential direction of the tire and the other portion of the sipe adjacent to the portion disposed at an offset position are connected to each other through a connecting portion extending in a direction intersecting the radial direction of the tire.
The amplitude of the sipe in the direction of the width of the sipe may change in the direction of the depth of the sipe, and the change in the amplitude occurs at one or a plurality of node positions disposed in the sipe in the direction of the depth of the sipe.
In the second aspect of the present invention, the pneumatic tire comprising a tread which comprises: block-shaped land portions divided by a plurality of main grooves extending in the circumferential direction of the tire and by many lug grooves intersecting the main grooves, and a plurality of sipes formed on the block-shaped land portions; wherein at least one portion of the sipe in the direction of the depth of the sipe has an amplitude in the direction of the width of the sipe and extends in the transverse direction of the tire, and the amplitude of the sipe in the direction of the width of the sipe changes in the direction of the depth of the sipe, the change in the amplitude alternating from an increase to a decrease or from a decrease to an increase at one or a plurality of node positions disposed in the sipe in the direction of the depth of the sipe.
In other words, at some position in the direction of the depth, the sipe in the pneumatic tire of the present invention has a bending portion which is formed by the portion disposed at an offset position in the circumferential direction of the tire in the first aspect of the present invention, and by the change in amplitude in the direction of the width of the sipe in the second aspect of the present invention. Therefore, the sipe of the present invention has an increased area of contact between the side faces of the sipe and exhibits an increased effect of suppressing bending down of the block-shaped land portion when acceleration or braking is applied in comparison with sipes having a conventional shape which extends straightly in the direction of the depth.
The bending down of the block-shaped land portion under application of acceleration or braking can be more effectively suppressed by the sipes in the present invention than by conventional sipes even when the number of the sipe increases because the area of contact between the side faces of the sipes and the force of contact between the sipes under deformation by compression force increase by the effect of the amplitude.
Therefore, in the pneumatic tire of the present invention, the increase in deformation of the block-shaped land portion due to the increase in the number of the sipe can be suppressed and deterioration in the road holding property can be suppressed. Thus, the pneumatic tire of the present invention can exhibit improvements in all of the grip performance of the tire on wet surfaces, the acceleration and braking performance on ice and the resistance to irregular wear.
In the third aspect of the present invention, the sipe substantially bent in the direction of the depth in accordance with the first aspect described above is characterized in that a cross-section of the sipe in the circumferential direction of the tire has a shape such that an offset shape in the region in the circumferential direction of the tire at one side of the central line of the block-shaped land portion in the circumferential direction of the tire and an offset shape in the region in the circumferential direction of the tire at the other side of said central line correspond to each other.
In the present aspect, the cross-section of the sipe in the circumferential direction of the tire may have a shape which is substantially symmetrical with respect to the central line of the block-shaped land portion in the circumferential direction of the tire.
In accordance with the third aspect, deformation of small blocks which are divided by the sipes is suppressed by the force of contact between the side faces of the small blocks at both sides of the sipe and thus the effect of suppressing deformation of the block-shaped land portion further increases.
In the fourth aspect of the present invention, the cross-section of the sipe in the circumferential direction of the tire has a shape such that the sipes adjacent to each other have shapes which are substantially symmetrical with respect to the central line between the sipes in the direction of the depth of the sipes.
In accordance with the fourth aspect, deformation of small blocks is suppressed by the force of contact between adjacent small blocks. In accordance with the present aspect, small blocks disposed at every other positions has a greater width around the bottom and show a greater rigidity and the deformation is more effectively suppressed. The deformation of the block-shaped land portion is suppressed with an improved balance.
In the fifth aspect of the present invention, the cross-section of the sipe in the circumferential direction of the tire has a shape such that a bending portion having a substantially bent shape is disposed in the sipe at a position in the direction of a depth which differs from sipe to sipe.
In accordance with the fifth aspect, for example, when each sipe has a single bending portion, the sipes may be formed in a manner such that the closer the position of the sipe to the central line in the circumferential direction of the tire, the deeper in the direction of the depth the position of the bending portion having a substantially bent shape. When the sipes are formed in this manner, rigidity of bending of the block-shaped land portion gradually increases from the central line in the circumferential direction of the block toward the side face of the block-shaped land portion open to a lug groove. Therefore, the present construction of the sipes shows an improved effect of suppressing bending down than conventional constructions of sipes.